1. Field of the Invention
The present invention relates to an image forming apparatus applied to, e.g., a printer or a plotter, which is capable of continuously recording a desired image on sheets of various sizes.
2. Description of the Related Art
Conventionally, the following two methods are known as methods of forming images on sheets of various sizes (e.g., sheets of A sizes such as A4, A3, A2, A1, and A0).
The first method is to set only an A0-size sheet roll (which is formed by winding an A0-size sheet into a roll) in an image forming apparatus and cut the sheet into a desired size after an image is formed. The second method is to set a plurality of sheet rolls of various sizes in an image forming apparatus and cut each sheet into a desired length after an image is formed.
To form desired images on sheets of various sizes from A4 to A0 by using these methods, three sheet rolls with different widths are necessary. More specifically, a 297-mm wide sheet roll corresponds to A4 and A3 sizes, a 594-mm wide sheet roll corresponds to an A2 size, and a 841-mm wide sheet roll corresponds to A1 and A0 sizes. Assuming the length of one sheet roll is 150 m, an A0-size sheet roll generally has a weight of about 10 kg.
To be able to mount these three sheet rolls, in conventional image forming apparatuses three drawer trays are provided in the lower portion or on the operation side of an apparatus. Each drawer tray consists of a holder for holding a sheet roll and a roller for pulling out a sheet from the sheet roll held by the holder.
In conventional image forming apparatuses of the above sort, a desired sheet pulled out by the roller is passed through a sheet path, conveyed to a recording unit by a predetermined conveying means, and subjected to image recording. The sheet on which an image is recorded is cut into a predetermined length by a cutting means arranged downstream in the sheet conveyance direction. Thereafter, the sheet is rewound and the next image recording operation is started.
In these conventional image forming apparatuses, however, a large space is required to arrange the three drawer trays on the operation side, and a large area on the operation side is occupied by these drawer trays. As a consequence, it is necessary to arrange an image-recorded sheet delivery tray on the non-operation side away from the operation side. This makes operations in one direction (operation side) of an apparatus difficult.
Also, when a sheet jam occurs in the conventional image forming apparatuses, it is necessary to pull out the corresponding drawer tray and this is troublesome for a user. Additionally, the sheet may be torn off halfway when the user pulls out the drawer tray. Even when the user can pull out the desired drawer tray, to release the portion where the sheet is nipped he or she must go around to the non-operation side of an apparatus and open a sheet jam management door.
To eliminate these inconveniences, it is possible to equip each drawer tray with a cutting means. However, this increases the dimensions and the cost of an apparatus. Furthermore, if all drawer trays are pulled out, the stability of the apparatus decreases.
To solve these problems, if an apparatus is so designed that all drawer trays cannot be simultaneously pulled out, the operability of the apparatus suffers.
In the conventional image forming apparatuses, a heavy sheet roll is set in the lower drawer tray of an apparatus. This makes the posture which the user takes when mounting the sheet roll unnatural and thereby increases the burden of the user when he or she sets the sheet roll. In addition, the use of drawer trays increases the sheet conveyance distance to the recording unit, and this increases the time required for a sheet to reach the recording unit. Also, a large error appears in the sheet position when the sheet passes through the recording unit.
In the conventional image forming apparatuses, a sheet detecting means detects a plurality of portions on the sides of a sheet while the sheet is slightly moved back and forth, thereby checking whether the sheet is skewed. However, the moving length of a sheet in the conveyance direction is very small compared to the width of the sheet. This makes accurate detection of a skew difficult. In addition, after the skew detection is performed, a recording means moves above the sheet to check whether the dimension in the sheet widthwise direction is appropriate. If the sheet has a slack or the like, therefore, the recording means may contact and damage the sheet in some cases.
In conventional image forming apparatuses using an inkjet recording method, the time required for ink droplets to reach a sheet from a recording head of a recording means varies if the spacing between the recording head and the sheet is not maintained constant. Since a carriage is so controlled as to perform printing while moving, if this variation occurs the positional accuracy with which ink droplets adhere to a sheet decreases.
In the conventional image forming apparatuses, recording is done by using a platen roller or a guide downstream of the platen roller as a platen surface. If the recording width of a recording means is large and a high recording speed is necessary, the radius of curvature of the platen roller cannot be ignored. Accordingly, a platen roller with a large diameter is necessary, with the result that a large space is required to accommodate this platen roller and the size of the apparatus is increased.
When a sheet is supported by the guide downstream of the platen roller, the sheet may sometimes float from the guide. Also, it is difficult to maintain the accuracy of the guide constant throughout the width because of the influence of parts accuracy and thermal expansion by environments. For example, in a method of conveying a sheet by drawing the sheet to a belt by suction, it is difficult to draw the sheet to the belt by suction with no slip because of the influence of inertia of the sheet or the like. There is another problem that the conveyance of the belt is not constant due to slip or the like cause. Furthermore, in a method in which a platen board is formed by drawing a sheet to a suction box by suction, a load is applied on the sheet while the sheet is being conveyed and consequently the sheet sometimes buckles during the conveyance.
When a sheet is also conveyed downstream of the recording unit, image recording cannot be performed until the sheet reaches a conveying means on the downstream side, resulting in a low recording efficiency. Additionally, since a non-recorded portion is formed on the leading edge of a sheet, the effective recording area is restricted.
In image forming apparatuses using a method of performing recording by reciprocating a carriage, if the width of a sheet to be conveyed is small, the time required for the carriage to return at the side portion of the sheet becomes longer than an actual recording time. This makes efficient image recording impossible.
In image forming apparatuses in which recorded sheets are stocked by sorting them in accordance with their sizes by using a sorter or the like device, if sheet sizes are large the space occupied by the sorter itself is increased. This makes the apparatuses of this type inconvenient in actual use. On the other hand, in apparatuses in which sheets are stocked by dropping them into a stocker, lower sheets are smashed to wrinkle by the weights of sheets falling on them.
Also, coated sheets are primarily used as sheets for an inkjet method, and these coated sheets easily form paper dust when cut. A large quantity of dust particles adhere particularly to the cut surface of a coated sheet or a cutter. Consequently, when the sheet is cut or when it is rewound or again fed after being cut, dust particles scatter in an apparatus and adhere to a recording head. When dust particles thus adhere to the recording head, it is no longer possible to normally eject ink. This problem of paper dust is significant in an apparatus having a cutting means above a recording unit.
Moreover, in the conventional image forming apparatuses, the set positions of sheet rolls are determined and hence it is not possible to flexibly change the set positions in accordance with the use condition or the objective of use. For example, it is impossible to set only a large-size sheet and record a large image on the sheet or to change the set position of a small-size sheet roll to a desired position.